(i) Field of the Invention
The present invention relates to a porous rubber stamp material having open cells for use in a private seal, an address seal, a date seal and the like. More specifically, it relates to a sponge rubber stamp material having numerous open cells from which a rubber stamp is made, and by this rubber stamp, an ink can be beforehand occluded, so that a seal can be put without using any ink pad.
(ii) Description of the Related Art
As stamps which can be used to continuously put a seal without using an ink pad every sealing operation, the stamps made of a sponge rubber stamp material having numerous open cells have been spread.
For example, Japanese Patent Publication No. (Sho) 48-41936 discloses a method for obtaining a rubber stamp by superposing a sheet A upon another sheet B, followed by vulcanization.
That is to say, a material rubber is mixed with a vulcanizing agent, a filler and a high-temperature closed cells-forming agent, and sodium chloride and sodium naphthalenesulfonate-formalin condensate are further added thereto, followed by mastication, to prepare the sheet A. On the other hand, the above-mentioned material rubber is mixed with the vulcanizing agent, the filler, a low-temperature closed cells-forming agent, an intermediate-temperature closed cells-forming agent and the high-temperature closed cells-forming agent, and sodium chloride and sodium naphthalenesulfonate-formalin condensate are further added thereto, followed by mastication, to prepare the sheet B.
Next, the sheet A is closely superposed upon the sheet B, and the laminate is then placed in a mold having an inside bottom surface provided with relief characters, and a pressure of about 200 kg/cm.sup.2 is applied to the mold so that the sheet A may be pressed against the relief characters. Afterward, vulcanization is carried out at a gradually rising temperature, and a molded article is then released from the mold, washed until sodium chloride or sodium sulfate and sodium naphthalenesulfonate-formalin condensate are removed, dehydrated, and then dried to prepare the rubber stamp.
Furthermore, Japanese Patent Publication No. (Sho) 59-28193 discloses that the sheet A is formed by adding sodium chloride, sodium sulfate or sodium nitrate of 200 to 350 mesh and soluble potato starch of 800 to 200 mesh pass to an unvulcanized rubber blend, and the sheet B is formed by adding sodium chloride, sodium sulfate or sodium nitrate of 32 to 100 mesh and soluble potato starch of 150 to 250 mesh to the unvulcanized rubber blend, and these sheets are then treated in the abovementioned manner to prepare a rubber stamp.
These methods all comprise a technique in which the sheet A is brought into contact with the mold having the relief characters, and the sheets are then pressed between hot platens to form the rubber stamp.
Most of these conventional porous rubber stamps are each prepared by laminating a print layer having a spring hardness of 20 to 30 (a measured value by a spring hardness testing machine; THE SOCIETY OF RUBBER INDUSTRY, JAPAN STANDARD SRIS 0101-1968) on the surface of an ink-occluding layer having a spring hardness of 10 to 20. Thus, when an excess push pressure is applied to the print member at the time of sealing, a print surface is crushed and the occluded ink excessively flows out, with the result that an imprint blurs and any sharp imprint cannot be obtained.
In addition, another porous rubber stamp material is also on the market in which a skin layer is formed on the surface of the print layer to decrease the diameter of cells through which the ink flows out. However, the stamps manufactured from this porous rubber stamp material have a noticeable unevenness, and when the excess push pressure is applied to the stamp at the sealing, the print surface is crushed and the occluded ink excessively flows out, so that the imprint blurs and any sharp imprint cannot be obtained. Conversely, the ink does not sufficiently flow, though no blur is present, and hence, when a sealing operation is continuously carried out, the problem of scratch inconveniently occurs soon.